Solid state lasers, such as Nd-doped YAG (Nd:YAG) lasers and Yb-doped YAG (Yb:YAG) lasers, Yb-doped fiber lasers, and Er-doped fiber amplifiers, etc. employ a semiconductor laser as their pumping source.
Conventional semiconductor lasers are disclosed, e.g., in Japanese Laid-Open Patent Publication No. 2004-165383, etc. In typical conventional semiconductor lasers, the width of the diffraction grating perpendicular to the length direction of the resonator is uniform along the length of the resonator, as described on page 28 and shown in FIG. 11, etc. of this publication.
Japanese Laid-Open Patent Publication No. H06-313818 discloses a semiconductor laser configured in such a manner that the diffraction grating has a dimension varying periodically along the length of the resonator. However, since the diffraction grating in the semiconductor laser described in this publication extends a relatively long distance along the length of the resonator, the semiconductor laser does not fall into the category of a DBR semiconductor layer, in which a diffraction grating is provided only in the vicinity of the emitting facet of the resonator.
Broad area semiconductor lasers permit a plurality of horizontal transverse modes which extend within a plane perpendicular to the crystal growth direction in a direction perpendicular to the length direction of the resonator. More specifically, in the case of longitudinal modes in a broad area semiconductor laser, they can be controlled by use of a diffraction grating so that the lasing wavelength is equal or approximately equal to the wavelength of the single longitudinal mode.
In the case of horizontal transverse modes, however, a number of modes including low order modes to high order modes appear in the semiconductor laser, since there is little difference between the reflectances of the diffraction grating in these modes. As a result, conventional broad area semiconductor lasers lase in many horizontal transverse modes, making it difficult to condense the light without the need for an additional complicated optical system.
The present invention has been made to solve the foregoing problem. It is, therefore, an object of the present invention to provide an improved semiconductor laser which lases in lower order horizontal transverse modes.